TBC instances sorted by difficulty and players cap:
If things are the same level gap, I have prioritized them to what I am guessing the difficulty will be (such as I believe Lady Vashj will be tougher than Magtheridon).
5 mans instances:
Hellfire Citadel - Ramparts - Level 60-62
Hellfire Citadel - Blood Furnace - Level 61-63
Coilfang Reservoir - The Slave Pens - Level 62-64
Coilfang Reservoir - The Underbog - Level 63-65
Auchindoun - The Mana Tombs - Level 64-66
Auchindoun - Auchenai Crypts - Level 65-67
Caverns of Time - Escape from Durnholde - Level 66-68
Auchindoun - Sethekk Halls - Level 67-69
Coilfang Reservoir - The Steam Vault - Level 68-70
Hellfire Citadel - The Shattered Halls - Level 70
Caverns of Time - Opening the Dark Portal - Level 70
Tempest Keep - The Mechanar - Level 69-70
Tempest Keep - The Botanica - Level 70
Auchindoun - The Shadow Labyrinth - Level 70
Tempset Keep - The Arcatraz - Level 70
10 mans instances:
Karazhan - Level 70
25 mans instances:
Blade's Edge Mountains - Gruul's Lair - Level 70
Hellfire Citadel - Magtheridon's Lair - Level 70
Coilfang Reservoir - Serpentshrine Cavern - Level 70
Tempest Keep - The Eye - Level 70
Caverns of Time - Battle for Mount Hyjal - Level 70
Shadowmoon Valley - The Black Temple - Level 70
Info about keys I'v gathered during my time on the beta server. Some info could be a bit wrong and some things might change untill release, if you have more info, please post.
HEROIC MODE KEYS
(Everyone in the party needs this key to be able to play this heroic instance)
Requires Lower City - Revered
"Unlocks access to Heroic mode for Auchindoun dungeons."
Sells in: Shattrath City, Lower City
Flamewrought Key (Everyone in the party needs this key to be able to play this heroic instance)
Requires Honor Hold - Revered
"Unlocks Heroic Difficulty for Hellfire Citadel dungeons."
Sells in: Hellfire Peninsula, Honor Hold (Alliance)
Key of Time (Everyone in the party needs this key to be able to play this heroic instance)
Requires Keepers of Time - Revered
"Unlocks Heroic Difficulty for Caverns of Time dungeons."
Sells in: Tanaris, Caverns of Time
Reservoir Key (Everyone in the party needs this key to be able to play this heroic instance)
Requires Cenarion Expedition - Revered
"Unlocks Heroic Difficulty for Coilfang Reservoir dungeons."
Sells in: Zangarmarsh, Cenarion Refuge
Warpforged Key (Everyone in the party needs this key to be able to play this heroic instance)
Requires The Sha'tar - Revered
"Unlocks Heroic Difficulty for Tempest Keep dungeons."
Sells in: Shattrath City, Terrace of Light
OTHER INSTANCE KEYS
Shattred Halls Key (only one in the party needs this key to be able to play the instance)
Kill a Blacksmith orc outside The Black Temple
Loot the "this-begins-a-quest"-item from him and take:
-Entry into the Citadel (Completes in Honor Hold for Alliance)
-Grand Master Dumphry (Completes in Honor Hold for Alliance)
-Dumphry's Request (4 Fel Iron Bar, 2 Arcane Dust, 4 Motle of Fire
-Hotter then Hell (kill a Fel Reaver in Hellfire peninsula)
Questreward for the quest "Hotter then Hell" is the Key to Shattred Halls
Shadow Labyrinth Key (only one in the party needs this key to be able to play the instance)
Unlocks the gate to Shadow Labyrinth
Drops from the Last boss in Sethekk Halls
Key to the Arcatraz (only one in the party needs this key to be able to play the instance)
"Unlocks the Arcatraz satellite of Tempest Keep."
Questline: Starts in Netherstorm, Stormspire
-Triangulation Point one
--Triangulation Point two
---Full Triangle (elite)
----Special Delivery to Shattrath City
-----Breaking in to Arcatraz (Completes in The Botanica and The Mechanar)
The Master's Key (Everyone in the raid needs this key to be able to play the instance)
Unlocks the Karazhan Instance
Questline: Starts just outside Karazhan
--Contact from Dalaran
----Entry Into Karazhan (Completes in Shadow Labyrinth)
-----The Second and Third Fragments (Completes in The Steamvault and Arcatraz)
------Can be more quests here, dont know yet
Tempest Keep, Eye of the Storm key
Questline starts in Shadowmoon Valley at a guy named "Oronok Torn-heart"
First you do some small quests for him, then he sends you out to find his 3 sons.
After you have found them and done all quests you get a quest to kill a golemagg-lookalike (5 man group)
When you have done that Oronok Torn-heart sends you to Shattrath City and there you can
pick up 3 quests and in all 3 it stands like this:
"Four trials you must pass before you are deemed worthy of the Tempest key"
"Trial of the Naaru: Mercy (Heroic)" Completes in Shattred Halls, Heroic
"Trial of the Naaru: Strength (Heroic)" Completes in Steamwault and Shadow Labyrinth, Heroic
"Trial of the Naaru: Tenacity (Heroic)" Completes in Arcatraz, Heroic
After all this is done, you get the Tempest Key
Caverns of Time, Mount Hyjal questline
First you need to save Thrall in Durnholde keep after that is done you can do Black Morass
In Black Morass you can take a quest from a guy in the beginning of the instance and when
you have killed the last boss in that instance and completed it, you can take the quest:
"The Vials of Eternity (raid)" just outside Black Morass.
Completes in Eye of the Storm(Tempest Keep) and Serpentshrine Cavern(Coilfang Reservoir)
After this is done, you will most likely get the key to Mount Hyal
Serpentshrine Cavern Questline
Halfway through The Slave Pens Heroic (Coilfang Reservoir) there was a guy that gave me the quest: The Cudgel of Kar'Desh and the quest reward is:
"The following Spell will be cast on you:
50000 yd range
Allowes the wearer to enter Serpentshrine Cavern: Lair of lady Vashj"
Quest Objectives: Loot the Earthen Signet from Gruul in Gruuls Lair and the Blazing Signet from Nightbane in Karazhan
Shadowmoon Valley, Black Temple
...... Copie .... GILDENSEITE.....